Vibrations are becoming increasingly significant for the design and configuration of engine vehicles and internal combustion engines. It is attempted inter alia, to influence in a targeted manner and model the sound generated by the internal combustion engine. Measures in this context are also combined under the term sound design. Such development work is also motivated by the realization that the customer's decision to purchase a vehicle is influenced to a significant and increasing degree, even decisively, by the sound of the internal combustion engine or of the vehicle. For example, the driver of a sports car prefers a vehicle or engine whose sound emphasises the sporty character of the vehicle.
In the scope of a sound design, vibrations are compensated, e.g., eliminated or extinguished, or individual vibrations of a specific frequency are isolated, filtered out and, if appropriate, modelled.
The following can be differentiated as noise sources on an engine vehicle: flow noise, noise due to the emission of solid-borne sound, and noise due to the introduction of solid-borne sound into the vehicle bodywork via the engine mount.
Flow noise includes, for example, the noise at the mouth of the exhaust, the intake noise and the noise of the fan, while the noise due to the emission of solid-borne sound includes the actual engine noise and the emission of the exhaust system. The engine structure which is made to oscillate by shocks and alternating forces irradiates the solid-borne sound as air-borne sound via its engine surfaces and in this way generates the actual engine noise.
The introduction of solid-borne sound via the engine mount, in particular the introduction of solid-borne sound into the vehicle bodywork, is of particular significance for the acoustic driving comfort.
The internal combustion engine and the associated secondary assemblies are systems are capable of oscillating and whose oscillatory behavior can be influenced. The most relevant components with shock excitation and force excitation are the crank casing, the cylinder block, the cylinder head, the crank drive, the piston and the valve drive. These components are subject to the mass forces and gas forces. The crank drive comprises here, in particular, the crankshaft, the piston, the piston pin and the connecting rod and forms the system which is capable of oscillating which is relevant for the method according to the disclosure.
The crankshaft is made to undergo rotational oscillations by the rotational forces which change over time and which are introduced into the crankshaft via the connecting rods which are coupled to the individual crank pins. These rotational oscillations give rise both to noise due to the emission of solid-borne sound and to noise due to the introduction of solid-borne sound into the bodywork and into the internal combustion engine. When the crankshaft is excited in the natural frequency range, large rotational oscillation amplitudes may occur which can even lead to fatigue fracture. This shows that the oscillations are of interest not only in conjunction with a sound design but rather also with respect to the strength of the components.
The rotational oscillations of the crankshaft are transmitted in an undesirable fashion to the camshaft via the control drive or camshaft drive, wherein the camshaft itself also presents an oscillatory system and can cause other systems, in particular the valve drive, to oscillate. In addition, the oscillations of the crankshaft are introduced into the drive train, via which they can be passed on to the tires of a vehicle.
The rotational force profile at a crankshaft throw of a four stroke internal combustion engine is periodical, wherein the periods extend over two revolutions of the crankshaft. The rotational force profile is usually decomposed into its harmonic components by means of Fourier analysis in order to be able to make statements about the excitation of rotational oscillations. In this context, the actual rotational force profile is composed of a constant rotational force and a multiplicity of harmonically changing rotational forces which have different rotational force amplitudes and frequencies or oscillation rates. The ratio of the oscillation ni of each harmonic to the rotational speed n of the crankshaft or of the engine is referred to as the order i of the harmonic.
Due to the high dynamic load on the crankshaft as a result of the mass forces and gas forces, the designers attempted, when configuring the internal combustion engine, to implement mass balancing which is as wide ranging as possible, e.g., is optimized. In this context, the term “mass balancing” combines all the measures which compensate or reduce the effect of the mass forces toward the outside. To this extent, the method according to the disclosure for balancing the mass forces relates not only to the mass forces as such but also to the moments which are caused by the mass forces.
In this context an approach to the solution is targeted adjustment of the throw of the crankshaft, of the number and of the arrangement of the cylinders and of the ignition sequence in such a way that the best possible mass balancing is achieved.
A six-cylinder in-line engine can be balanced in this way. The six cylinders are combined in pairs in such a way that they run in parallel mechanically as a cylinder pair. The first and sixth cylinders, the second and fifth cylinders and the third and fourth cylinders are therefore combined to form a cylinder pair, wherein the crankshaft pins or crankshaft throws of the three cylinder pairs are each arranged offset by 120° CA on the crankshaft. Running mechanically in parallel means that the two pistons of the two cylinders which run mechanically in parallel are located at the same ° CA (degrees crank angle) at the top dead center (TDC) or bottom dead center (BDC). When a suitable ignition sequence is selected, the mass forces are balanced.
In the case of a three-cylinder in-line engine, the mass forces of the first order and the mass forces of the second order can also be balanced by selecting a suitable crankshaft throw and a suitable ignition sequence, but not the moments which are caused by the mass forces.
Complete mass balancing, as in the case of the aforementioned six-cylinder in-line engine, may not be implemented in every condition, with the result that further measures have to be taken, for example arranging counter weights on the crankshaft and/or equipping the internal combustion engine with at least one balancing shaft.
The starting point of these measures is that the crankshaft is loaded by the rotational forces which change over time and which are composed of the gas forces and mass forces of the crank drive. The masses of the crank drive, for example the individual masses of connecting rod, of the piston, of the piston pin and of the piston rings, can be transferred into an oscillating equivalent mass and a rotating equivalent mass. The mass force of the rotating equivalent mass can easily be balanced in terms of their external effect by counterweights arranged on the crankshaft.
The balancing of the rotating mass force caused by the oscillating equivalent mass is more complex, said mass force being approximately composed of a mass force of the first order, which rotates at the engine speed and a mass force of the second order which rotates at twice the engine speed, with higher order forces being negligible.
The rotating mass forces of any order can be virtually completely balanced by the arrangement of two shafts, referred to as balancing shafts, which rotate in opposite directions and are provided with corresponding weights. The shafts for the balancing of the mass forces of the first order rotate here at the engine speed and the shafts for the balancing of the mass forces of the second order rotate at twice the engine speed.
Even in the case of complete balancing of the rotating mass forces, mass moments can be produced since the mass forces of the individual cylinders act in the central planes of the cylinders. These mass moments can in an individual case again be compensated by a balancing shaft which is equipped with weights.
The moments caused by the mass forces of the first order, for example in the case of a three-cylinder in-line engine, can be compensated by a single balancing shaft which rotates at the engine speed in the opposite direction to the crankshaft and at whose ends two balancing weights which are arranged offset through 180° and serve as an unbalance are provided.
The provision of one balancing shaft or, if appropriate, a plurality of balancing shafts not only increases the spatial requirement and the costs but also the fuel consumption. The increased fuel consumption is caused, on the one hand, by the additional weight of the balancing unit, in particular of the shafts, and of the counterweights which serve as an unbalance and which perceptibly increase the overall weight of the drive unit. On the other hand, the balancing unit with its rotating shafts and other moving components contributes significantly to the friction loss of the internal combustion engine and to the increasing of this friction loss. The latter has relevance, in particular, due to the fact that the balancing unit is continuously operational as soon as the internal combustion engine starts and is operated. The mass forces are balanced continuously here without it being taken into account whether or not the instantaneous operating state of the internal combustion engine at all demands such mass balancing, for example for reasons of the noise design.
It would therefore be possible to dispense with balancing of the moments caused by the mass forces of the first order in a three-cylinder in-line engine at relatively high engine speeds since the noise caused by the oscillations is evaluated as being problematic only at low rotational speeds and during idling, and there is a risk of excitation in the region of the natural frequency only in this rotational speed range. On the other hand, at relatively high rotational speeds mass balancing may be dispensed with.
The inventors herein have recognized the above issues and offer an approach for mass balancing with relatively low friction loss. Accordingly, an internal combustion engine having at least one cylinder which is associated with a crank drive comprises at least one mechanically driven balancing unit for balancing the mass forces, the balancing unit including at least one balancing weight which serves as an unbalance by initially rotating a first mass about a rotational axis relative to a second mass when the balancing unit is operational, and an interrupter unit to disconnect the at least one balancing unit from the mechanical drive in a switched off state and connect it to the mechanical drive in a switched on state.
The internal combustion engine according to the disclosure uses, for the mass balancing, a switchable balancing unit which can be activated, e.g., switched on, when balancing is indicated but also deactivated, e.g., switched off, when balancing is not indicated, in order to reduce the friction loss.
The at least one balancing unit can be switched by virtue of the fact that according to the disclosure an interrupter unit is provided which disconnects the balancing unit, e.g., the mass balancing, from the mechanical drive for the purpose of deactivation. For this purpose, the force flux between the mechanically driven balancing unit and the mechanical drive has to be interrupted, for example by a clutch, which is opened in order to switch off the balancing unit.
In the case of a three-cylinder in-line engine it is possible in this way for mass balancing to take place at low rotational speeds and in the idling mode, with mass balancing being dispensed with toward relatively high rotational speeds by switching off the balancing unit, in order to reduce the friction loss and therefore the fuel consumption. In the case of mechanically driven balancing units, the balancing shafts which are used are generally or preferably arranged underneath the crank casing.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.